This invention concerns thermoforming methods and apparatus, and more particularly thermoforming parts using two sheets, each individually formed and the two pieces then fused together to complete the part. Thermoforming involves drawing a heated plastic sheet against a mold surface using vacuum and/or air pressure. In the twin sheet process, two pieces are thermoformed in a forming station, and upper and lower platens holding the two formed pieces are forced together to fuse the two pieces together and complete the part.
It is sometimes desirable to utilize a well known technique called xe2x80x9cpluggingxe2x80x9d for deep formed parts, in which a complementary shaped tool assists in thermoforming the part against a mold surface. That is, there is a male and a female part to the tooling, one the mold, the other the plug assist tool.
This technique produces more uniform wall thickness in deep formed parts.
Plugging is difficult to practice in conventional twin sheet thermoforming, as the need to locate the plug tools opposite the mold would prevent the separately formed pieces aligned at the forming station from being pressed together.
Special arrangements have been devised to allowing plugging to be practiced in twin sheet thermoforming processes.
U.S. Pat. No. 5,658,523 issued on Aug. 19, 1997 for a xe2x80x9cMethod and Apparatus for Forming Twin Sheet Hollow Plastic Articlesxe2x80x9d describes a process in which two sheets are loaded into a pair of side by side clamping frames, which are both simultaneously transferred by indexing of a rotary wheel carrier through a heating oven, and then to a forming station, where side by side molds and plug assist tools form each part piece. One of the molds, with one of the part pieces held therein is shifted laterally (after withdrawal of the plug assist tool) to lie beneath the other mold holding the other part piece, and the two pieces are fused together. The joined pieces are transferred to a cold forming station where the part is completed by a final forming operation.
The side by side simultaneous transfer of two clamp frames and sheets requires a large oven for heating both sheets at once, and also makes it difficult to carry out different degrees of heating for different sheet thicknesses.
Another disadvantage is that removal of a finished workpiece is conducted at the same location where one of the new sheets must be loaded, complicating the sheet load mechanism.
It is the object of the present invention to provide a twin sheet rotary transfer thermoforming apparatus in which a side by side simultaneous transfer of two sheets is not required when using plug assist tooling to form each part piece.
The above object and others which will become apparent upon a reading of the following specification and claims are achieved by loading sheets one at a time into each of a series of clamping frames disposed about a rotary transfer wheel, which indexes to bring each clamping frame into each of the processing stations, including preliminary and final heating stations, a forming station, and an unload-load station.
Each clamping frame is mounted on the transfer wheel station to be able to be located in either of two shifted positions on the wheel structure by action of a shuttle transfer drive shifting clamping frame between either of these two positions.
In the unload-load station, unloading takes place in one position of the clamping frame, and the clamping frame is then shifted to the other position where loading of a new sheet takes place, simplifying the design of the sheet loading mechanism.
Each sheet is then successively carried by indexing of the transfer wheel into one or more heating stations. Preferably, a preheat and final heat stations are provided to reduce cycle time.
The clamping frame is shuttled to the one position for the first of each two succeeding sheets indexed into the form station, so that sheet is disposed in one of two forming tooling sets, including a mold and a plug assist, whereat forming of the first part piece is carried out. The clamping frame releases the piece to allow that piece to remain in the mold and be retracted with the mold platen.
The next successive sheet is moved into the other of the tooling sets by the next wheel index, since its clamping frame remains in the one position on the wheel, and the second piece is then formed.
After forming, the top platen remains lowered and only the bottom platen is retracted, the second formed piece remaining clamped in its clamping frame. The top platen of the other tooling set is then transferred over the bottom mold of the one tooling set, carrying the piece and clamping frame with it. The two pieces are then fused by operation of respective press platens to form a completed part.
The finished part is then rotated to the unload-load station with the next wheel index, and the clamping frame releasing the part to allow it to drop onto a conveyor.
The clamping frame is then shuttled to the other shifted position of the wheel for loading of a fresh sheet.
The one at a time transfer of the sheets makes individualized heating of the two sheets easier and reduces the oven size required, while using a preheat oven reduces cycle time. Since part unloading and sheet loading take place at two different locations, the loading mechanism is simplified.